Our family grills every weekend in the summer and we all enjoy sitting around the fire bowl every cool evening. My father-in-law once asked himself whether we could add the ash to compost or perhaps use it to fertilize the vegetable patch directly. I promised him that I would research the topic and also share with you what I found out about wood ash.
First of all: not all ashes are the same. If untreated natural materials such as wood, straw or plant material are burned, the ashes can be used for compost or the garden. Briquettes, coal and other treated materials contain pollutants and have no place in the compost heap.
You should also be able to distinguish between the different types of ash. The ash from hardwood species is richer in nutrients than the ash from softwood species. The two types should therefore be dosed accordingly to prevent over-fertilization.
In cases of doubt: dispose of the ashes instead of using them. Treated wood often contains toxins that cannot be broken down during composting. They can damage ornamental plants and get into fruit of crops.
Composting wood ash: In these cases it makes sense
The ash from untreated wood contains the same nutrients as the rock dolomite. In addition, it is rich in potassium and phosphorus and is alkaline. Fertilization should therefore be tailored to the needs of the garden plants and take into account the composition of the garden soil.
So before you shake them into the compost heap, it's best to have the garden soil tested in a laboratory. Adding burned wood to compost makes sense in the following cases:
- With a low basic potassium content in the soil. This is often the case in nutrient-rich soils with a high sand content. The potassium content in moorland soil is also significantly lower than in normal garden soil.
- In light soils, the micronutrient is often washed out by spring rainfall, while heavy soils have higher amounts of potassium on average.
- A potassium deficiency can also arise in vegetable gardens that have been planted year after year with heavy feeders such as potatoes, pumpkins and tomatoes. A preventive measure in this case is to regularly incorporate compost with wood ash.
- For potted plants – in pots, flower boxes and tubs the soil is already poor in nutrients in mid-summer. This is because watering and rainfall wash away the nutrients in the soil.
- Whenever there are signs of potassium deficiency in the garden. The damage caused by the deficiency can be recognized quickly and easily: the edges of the plant leaves are dry, the fruits taste watery, the plants are less heat and frost resistant than usual, are weakened and often become sick.
- If the pH of the soil is acidic and needs to be increased. You can test the soil with litmus paper.
- Even with normal, permeable garden soil, wood ash can be safely composted.
Do not use ash as fertilizer in these cases
Of course, there are exceptions - in some cases adding ash to the compost heap is not recommended.
- For example, if the soil analysis shows that the amounts of potassium are sufficient.
- Or if you want to use the compost to fertilize plants that prefer acidic soil. Examples include blueberries, rhododendrons and azaleas.
- Compost with ash must not be used when sowing. Ash is rich in nitrogen, an important nutrient for plants, but it can inhibit seed germination. Ash can therefore only be used for young plants that have already sprouted and have formed at least two pairs of leaves.
Incorporate wood ash into the compost
The basic rule is: adding small amounts of ash to the compost heap every now and then can have a positive effect on the garden plants and the soil. So you shouldn't overdo it - regularly disposing of the barbecue ashes in the compost is out of the question.
The wood ash should be scattered evenly on the compost, ideally directly over green waste. Plant residues and grass clippings are well suited for this purpose. For example, you can first mow the lawn on the weekend, dispose of the clippings in compost and then incorporate the wood ash. This means it can release nutrients more quickly. The ashes should definitely be cooled down first, otherwise there is a risk of fire. It is best to first store the wood ash in a metal bucket for 2 days and only then compost it.
Can you put ash in the worm bin?
There is currently no scientific evidence as to whether ash in vermicompost is beneficial or harmful. However, it is effective against certain mites and is often added to the sand bath by pig farmers. So one could assume that it can cause damage in the worm bin. That's why most hobby gardeners avoid it and use primary rock flour instead.
